Such an internal combustion engine is disclosed in German Offenlegungsschrift 4,319,380 and can be used either for a diesel or an otto engine.
Exhaust-gas recirculation (EGR) is a known technique for reducing, in particular, the NO.sub.x emission of combustion engines. In the case of EGR, some of the exhaust gases are fed back to the inlet of the engine. Especially in the case of turbo-charged engines (engines provided with turbo-compressor) and turbo-charged engines with intermediate cooling (or intercooling; cooling of the compressed inlet air), this can be done in a number of different ways. The most well known are:
a) feedback of exhaust gas from downstream of the turbine to upstream of the compressor, PA0 b) feedback from upstream of the turbine to downstream of the compressor and PA0 c) feedback from upstream of the turbine to upstream of the compressor. PA0 a) compressor and intercooler are not exposed to the EGR gas contamination, and PA0 b) the mass flow through compressor and intercooler does not increase if EGR is used.
An advantage of design b) is that no contamination can occur of the compressor and the intercooler optionally connected downstream thereof.
A problem in the case of exhaust-gas recirculation is the metering of the quantity of exhaust gas in the inlet duct. This determines the emission of the respective internal combustion engine. In order to limit the NO.sub.x discharge, sufficient exhaust gas has to be recirculated. On the other hand, an unduly large addition has the consequence that the power of the internal combustion engine is adversely affected if this is at the expense of the quantity of air. With an excess EGR flow, the particle emission also increases. In addition, a pump loss is produced by the implementation of an EGR flow by the bleed points, which adversely affects the efficiency.
As a result of the efficiency loss, there are still hardly any EGR systems at present which can effect EGR in the high/maximum load range. To generate EGR in this range, use is fairly often made of a back-pressure valve in the exhaust system. In general, this results in a significant increase in fuel consumption.
EGR systems are in fact frequently used in the lighter diesel engines. As a result of the official exhaust-gas emission test procedures for this category of engines, where the stress is on low and moderate engine load, it is possible to make do with EGR at low and moderate engine load. In this case, `replacement` EGR is also involved; some of the inlet air is replaced by EGR gas. In the case of low/moderate load, this generally produces no substantial deterioration in the performance because, even after replacement, an adequate excess of air is still present. In these systems, it is generally possible to make do with fitting a simple throttle valve (between compressor and EGR introduction location) in order to effect the required EGR flow.
In the design according to German Offenlegungsschrift 4,319,380, a regulable back-pressure valve is sited downstream of the turbine. As this back-pressure valve is further closed, the back-pressure upstream of the turbine will also increase, as a result of which (more) exhaust gas flows towards the inlet via the bleed. Since turbine work and compressor work do not change or hardly change as a result of this design and in view of the usual compressor characteristic, the (fresh) air flow through the engine will drop. This is `replacement` EGR. As a result of the consequently lower excess of air, the efficiency of the engine will drop and the smoke emission will increase.